Process for the preparation of methacrylic acid



' A Pate nlet! rnnrs'nfsfnoiw or cm V GI-mOA Mummy, m to This invention relates-to a process for" the oxidation of methacrolein' (u-methyl acroleiii) 'to form methacrylic' acid and particularly to the use of certain catalysts suitable for increasing the;

5' rate of the oxidation. Methacrylic acid has been mace in the past 'by several methods one of which is to treat acetone with hydr v a id t produce acetonecyanohydrin and then to hydroivze and dehydrate this 19 product to form the It has also been made by splitting hydrogen chloride out of p-chlorisobutyric acid. r

It has now been found that methacrolein, ob-

tainable by dehydrogenation or oxidation of s 18 methyl allyl alcohol, can be oxidiied to 'metha crylic acid by means of oxygen 'or a gas containing oxygen and that thk process maybe accelerated by the addition oi soluble salts of j certain metals, such as copper, nickel, cobalt $0 and manganese, to the-reaction mixture. It is known that compounds like methacroiein and methacrylic acid polymerize readily and that oxygen will accelerate this polymerization. Inorder to obtain satisfactory yields of the methacrylic acid it is necessary to the ten" dency to polymerize and it has-been found that certain polymerization retarders may beadded to the reaction mixture so that, during the oxide.- tion, practically no polymerization takes place.

- Furthermore, the reaction may be carried out'in the presence of organic solvents in order to facilitate the control of theoxidation. Water,

however, should not be present salt has a detrimental efiect on the reaction.

The present process is carried out in general by dissolving the methacrolein in inertliquid, adding thereto a suitable catalyst or polymerization retarder or both and passing oxygen, air or other oxygen-containing-gas into the solution at 40 any desired pressures from below to above atmospheric. It is preferable, however, to-use super- ,atmospheric pressure which maybe *as high as twenty atmospheres or more. 4

Suitable solvents for this process must not be aflected by the oxygen nor by the-catalysts used.

such solvents are principally the low boiling hydrocarbons or the aliphatic and aromatic series,

and theirvchlorinated derivatives as. for example, benzene, gasoline, carbon tetrachloride, trichloro- 1 ethylene, ethylene dichloride, etc.

'. The catalystsbest'suited iorthewm i are salts or copper, nickel, cobalt and manganese and preferably salts of organic ecidssuch as oleic, stearic, etc. The acrylates and niethacrylates'oi 55. these metals-are particularly suitable.

No. 88,089. ln'flermsny July Joana-m) 'inseparatebatches. Inbothcasesitlsad- 'vention' which,. however, is not limited to the June a,

The process is best carried out at. temperatures I between 20 and C. althoughlower or higher temperatures maybe employed if desired. Ther'elativeamounts oi'methacrolein and solvent may vary over a wide range. It is possible to use as much as of methacrolein in the mixture but for-commercial production it is preferable to keep the concentratlon below about 50% and concentratlons of 10 to 25% have been found toglve very satisfactory results, particularly in large scale 10 operations.

In all cases water should be excluded from the reaction mixture as far as possible. Thus, anhydrous solvents should bechosen and preferably those whichcan'dissolve or absorb only very small amounts of water. It is permissible to have organic hydroxy compounds present although their amount shouldbe smal l as otherwise they have an unfavorable effect on the oxidation v process. Small amounts of the organic hydroxy compound may exert a favorable influence on the reaction by retarding the polymerization of the metha--' cr'olein and methacrylic acid. Amounts of alcohols ranging from 5 to 20% may be used without any detrimental effect. Other polymerization retarders such as sulfur and copper may be used during the oxidation process but organic retarders such as phenol, hydroquinone, etc., are best used after oxidation is completed in order to prevent polymerization of the methacrylic acid. when alcohols are present, it is possible to esterify the acid formed in the same reaction mixture. The oxidation may also be made more eflectlve by insuring an intimate contact between the gas and the liquid by agitation, spraying or by flowing the liquid over large sm'faces so. as to expose as much as possible of the liquid to the action of the oxygen. I

The process may be. carried out continuously or vantageous to have the solution of methacrolein in the continuous phase and to bring it into contact with the air in finely dispersed form.

The following examples will illustrate the inexact time, temperature, concentration, catalyst,

retarder, etc., shown as the invention may be otherwise practiced within the scope of the appended claims.

Example 1 AmlxtureoflkK. ofmethacrolein (n. P. ea!- c.) andi kg.ofbenzol,towhichhasbeenadded about 0.5% of pulverized sulfur and about 1% or is charged into 'a-r'eaction l5 .tower. Air is admitted at the bottom of. the

tower through a mule at the rate of about 1.5

liters per minute. The reaction mixture is preterably kept in constant circulation during the entire time. The air which passes through the solution is led into a reflux condenser in which any entrained vapors oi methacrolein are condensed and returned to the reaction mixture. At the end oi 12 hours about 80% of the methacrolein mlis converted to methaerylic acid which may be separated by fractional distillation. In order to prevent the polymerization oi the methacrylic acid during the'fractionation about 2% of hydroquinone is added.

u 7 sample 2 Y A 40% solution of methacrolein in benml is heated to 50 C. and treated with oxygen at a pressure 01 atmospheres for 10' hours. The solution is agitated during the entire-operation.

In this time '10 to 80% o! the methacrolein is 80 parts of a 50% solution of methacrolein in benzol, containing 2 parts of mostly undissolved copper acrylate. is heated for hours at 50 C.

under an oxygen premir'e of 2 atmospheres. I 90 to 95% ot the methacrolein is converted to methacrylic acid.

- trample 4 A mixture 01 10. partsoi methacrolein, 90 parts of toluol and 0.1 part of nickel acrylate are treatedat5,00.withon*gen atapressureoi2 atmospheres. Methacrylic acid is formed but at a somewhat slower rate than when benaol is used as the solvent. 1 O Example 5 A mixture of 20 parts of methacrolein and 20 parts oi methylene chloride containing 0.6 part oif copper acrylate and 5 parts of copper shavings is treated at C. with. oxygen under a pressure of about '2 atmospheres. At the end of 25 hours about 90% or the methacrolein is converted to methacrylic acid.

lmmple 6 50 A mixture oi 20 parts 0! methacrolein and parts or methylene chloride to which has been added ozpartotcopperacrylate and 2parts of copper shavings istreated at 40" to 50' C. with oxygen at a pressure of 5 atmospheres. At the end of 25 hours to of the methacrolein has been converted to methacrylic acid.

In place of the methylene OhJOl'ldQJhOWll in 5 the i'oregoing examples other chlorinated hydrocarbons may be used. for example, carbon tetrachloride, acetylene tetrachloride. etc.

In place 01' the nickel and copper acrylates shown in the foregoing examples cobalt or man- 10 ganese acrylate or the methacrylate oi copper. nickel, cobalt and manganese may be used with equally good results. 'Ihese catalysts are in general nearly insoluble or soluble only with diiliculty in the reaction mixture.

The methacrylic acid thus produced may be converted into its esters in any desired manner. It is advantageous to use a polymerization retarder, such as a phenol, an aromatic amine,

sulfur or copper, during the esteriilcation. Salts 20 of the acid may also be prepared and the ammonium salt can be converted to the amide by heating.'

I claim: I

1. The process of oxidizing methacrolein to as methacrylic acid which comprises treating the methacrolein in an inert anhydrous solvent with gaseous oxygen in the presence of an oxidation catalyst which is an organic salt of .a metal of the group consisting of copper, nickel, cobalt and 8 manganese.

2. The process of oxidizing methacrolein to methacrylic acid which comprises treating the methacrolein in an inert anhydrous solvent with gaseous oxygen in the presence 0! copper acrylate. ll

3. The process of oxidizing methacrolein to methacrylic acid which comprises treati g, the methacrolein in an inert anhydrous solvent with gaseous ongen at temperatures of from about 20 C. to about 60 C. and ongen pressures up 0 to about 20 atmospheres in the presence of an oxidation catalyst which is an'organic salt oi. a metal of the group consisting or copper, nickel. cobalt and manganese.

4. The process of oxidizing methacrolein to 45 methacrylic acid which comprises treating the methacrolein dissolved in benzol with gaseous oxygen inthe presence of copper acrylate at temperatures o! from about 20 C. to about 60 C. and

oxygen pressures up to about 20 atmospheres. so

; WALTER. BAUER. 

